1. Field
The embodiments described below relate to, in general, a communication system. More particularly, some embodiments concern frequency hopping communication systems that provide a mechanism for a communication device to dynamically adapt its hopping sequence and timing within a deterministic period of time to communicate with communication devices in a network of communication devices.
2. Description
A number of current wireless communication systems, including for example, Bluetooth, and WLAN, provide the ability for a network of associated communication devices to communicate with each other over a range of frequencies. The communication devices of the system may generally be categorized as one of an access point (AP) and a client. The AP generally acts, to an extent, as a network controller for the wireless network. The clients may be mobile devices or fixed devices that are equipped to communicate with the APs. In a frequency hopping communication system, each communication device includes a transceiver that changes frequencies in synchrony with the other communication devices of the network such that their communication takes place over a common sequence of different frequencies over time. The common sequence of frequencies visited by the sending and receiving communication devices is referred to as the frequency hopping sequence or the hopping sequence.
Some existing frequency hopping (FH) networks may generally be classified as either synchronized or unsynchronized. In synchronized FH networks, the hopping information (i.e., the sequence of frequencies and when to change frequencies) of a cell is known by the AP (and sometimes even clients) of a neighboring cell. In unsynchronized FH networks however, such hopping information is not necessarily known outside of a particular cell. In existing systems, the handoff of mobile clients (i.e., communication devices) in a synchronized FH system has been well-studied and there are two basic methods for achieving a fast handoff in synchronized FH networks. The first method involves the AP of each cell collecting information regarding the hopping sequence and timing of neighboring cells. Each client can then learn from its current AP the hopping information of neighboring cells. As a result, as it roams towards a new AP in a neighboring cell the client can precisely know which frequency to use and when to contact the new AP. According to the second method, the hopping sequence of neighboring access points (APs) is coordinated such that it is possible for a client to infer the hopping information of neighboring APs by knowing the identity of its current AP. As a result, there is no need for an AP to explicitly tell its clients the hopping information of neighboring APs.
For unsynchronized FH networks, the APs of different cells do not know the hopping information regarding the hopping sequences and timing of each other. In such existing cases, one of two approaches is generally used to achieve fast handoffs for roaming clients. In the first method, the hopping sequences of all APs are designed such that they will always include a time slot on one or a few special or control frequencies within any period of time of a particular duration. The exact timing of the special/control time slot need not be known to the client. This method guarantees a client listening on one of such special/control frequencies can find any AP in range, provided that such AP sends out a packet while it is operating during that particular time slot. For the second method, the hopping sequence of each AP is completely unrelated to other APs. Additionally, some APs may not even use the full set of available frequencies. As a result, in general, there is no way to guarantee that a client joining a cell can synchronize with the AP of the new cell in a finite amount of time unless there are some constraints placed on the hopping sequence.
It is noted that there are a number of variations of the different FH networks introduced above. For example, various degrees of synchronization may be used in some networks. Some synchronized FH networks have been classified as either “loosely” or “tightly” synchronized systems depending on the accuracy of synchronization for the network. Regardless of the classification however, both systems are similar in nature. Also, some FH networks may use passive listening while others may use active probing. With passive listening, each AP periodically sends out packets called beacons so that their presence can be discovered or verified by a client trying to synchronize with the AP. With active probing an AP does not necessarily send out beacon packets when there are no data to transmit. Instead, clients trying to discover an AP send out soliciting messages called probe packets to solicit information from APs. When an AP receives a probe packet, it responds with a reply packet to announce its presence to the probing client.
It is noted however that existing frequency hopping schemes have a number of disadvantages. For example, synchronized FH systems, regardless of the degree of synchrony required, are more complex and difficult to implement than unsynchronized FH systems because APs therein must inform other APs or clients of other APs of their current hopping sequence and timing. In some cases, the hopping sequences of APs change continuously over time, thereby making it impossible for those APs to keep other APs or clients continuously up-to-date with their hopping sequence. Also, not all APs may have a reliable communication link with other APs to facilitate fast and/or reliable updates.
While unsynchronized FH systems may be easier to implement than synchronized FH systems since each AP can choose their own hopping sequence independently of each other, unless certain restrictions are placed on the choice of hopping sequences or the hop timing, one cannot guarantee that a handoff can be completed in a fixed amount of time. For example, if an AP does not use a particular frequency due to a high level of interference on that particular frequency, then a client listening on that frequency will never discover the AP regardless of how long the client listens.